Demonstration Description: One aluminum tube and one plexiglas tube are mounted on a vertical stand. Two apparently identical bobs are dropped through the tubes at the same time. One bob takes several seconds longer to fall through. One bob is strong magnet. As it falls through the aluminum tube it induces an electric field which in turn generates a magnetic field of its own, slowing the fall of the magnetic bob. The bobs can be reversed and they fall at the same rate. In 26-100 the TV projection system has to be used.
1. Stand with aluminum tube and plexiglass tube
2. Magnetic bob and metallic bob
Drop the bobs through both tubes simultaneously and observe the time that it takes each to fall. Switch the bobs and try the same thing.
When either the magnetic or metallic bobs are dropped through the plexiglass tube, they fall with the same free fall acceleration. However, when the magnetic bob is dropped through the aluminum tube, it induces a current within the tube that creates an opposing magnetic field, consequently slowing the bob. This is because the falling magnetic bob creates a changing magnetic flux through the tube; the field is increasing below the bob and decreasing above the bob. Opposing fields are created below and above the bob by induced currents. This does not happen with the metallic bob.
See chapter 29, page 1004, University Physics, for information on electromagnetic induction and Lenz's Law.
Young, Hugh D., Roger A. Freedman, and A. Lewis Ford. Sears and Zemansky's University Physics: With Modern Physics, 12th ed. San Francisco: Pearson Addison-Wesley, 2008.
An alternate demonstration involves a substantial copper "ring" positioned
halfway up and concentric to a clear plexiglass tube which stands perpendicular
to the lecture table. A very strong rare earth magnet is then dropped into
the top of the tube and falls quickly until it nears the copper ring, at which
point its descent is markedly slowed as opposing magnetic fields are induced
in the ring. After passing through the ring the magnet quickly dtops down the
remainder of the tube. The copper ring is often then immersed in liquid nitrogen
until at temperature, then placed back on the tube and the demonstration is
repeated. The braking effect is at this point strikingly more pronounced due
to the decreased resistance in the copper to the induced currents.
Note 1: There is a small wooden platform with an attached wooden pedestal
and foan padding which should be used to prevent damage to the magnet.
Note 2: The copper ring takes a few moments to be completed cooled by the
liquid nitrogen. It has reached equilibrium when the liquid nitrogen briefly
a flurry of increased boiling and then ceases to boil completely. This might
take up to two minutes depending on the initial temperature of the copper ring.
Also as this copper ring is particularly massive, it is best to do the warm
trial first as it will take up to 10 or 15 minutes to return to room temperature
after cooling completely.
Brave lecturers may put the ring in a back pocket, etc to speed the warming
but this risks discomfort and frostbite.